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In lots of applications water or gas-tight seals can easily be formed by tightening two surfaces together. These often consist of crudely made and inexpertly assembled parts, yet they seal so perfectly that a molecule of air cannot pass through. At a fundamental or molecular level how does this work?

One example is threaded tapered pipe (e.g. NPT in the USA) which tightens to a form seal apparently as the thread tips deform under pressure. Threads can be cheaply made, rusty, etc. yet still work correctly.

Another example is when you tighten a flexible gasket between two metal pieces; the gasket deforms and makes the seal. Like a garden hose.

Thanks

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  • $\begingroup$ NPT needs some kind of sealant to work—Teflon tape or pipe joint compound. $\endgroup$ – Ben51 Nov 24 '18 at 14:03
  • $\begingroup$ @Ben51 I've read that but also read conflicting information such that the "sealant" is really just a lubricant which allows the metal-to-metal contact to be made with sufficient pressure. But both could be right, all types of "sealant" may not necessarily work on the same principle. $\endgroup$ – UuDdLrLrSs Nov 24 '18 at 18:09
  • $\begingroup$ Interesting. I always assumed the Teflon fills the gaps. $\endgroup$ – Ben51 Nov 24 '18 at 18:16
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The deformability of the materials at the mating surfaces of the joint, plus the ability to apply progressively increasing compressive stresses on that joint, is what does the trick.

The best example of how an imperfect gasket can furnish an airtight seal between two imperfectly shaped metal parts is the case of a rubber ring trapped between two metal pipe flanges, cut so they interlock. The initial seal is provided by clamping the flanges together with bolts, causing the rubber ring to flatten. Then, when internal pressure is applied to the pipes, that pressure strives to drive the rubber ring deeper into those regions where the seal is poor. By "capturing" the rubber ring in this manner, the quality of the seal improves with increasing pressure and hence furnishes an almost perfect (airtight, or watertight) seal.

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